Extraction of Benzoic Acid

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The purpose of this experiment is to practice common organic laboratory techniques inside the lab to get one oriented to the basic methods of procedure that can be used for later experiments. This experiment involves the separation of benzoic acid from a more crude form, consisting of benzoic acid, methyl orange, a common acid/base indicator, and cellulose, a natural polymer of glucose (Huston, and Liu 17-24). The technique that is used to perform this separation is called extraction.

Extraction is a systematic process of separating mixtures of compounds, taking advantage of the affinity differences of compounds to separate them (Padias 128-37). This technique recognizes the principle that “like dissolves in like,” that is, polar solutes dissolve in polar solvents and non-polar solutes dissolve in non-polar solvents. Through extension of this principle, one can use extraction to separate compounds of a mixture. There are three different methods of extraction; solid-liquid extraction, liquid-liquid extraction, and chemically active extraction. Solid-liquid extraction is used when a desired compound, in a solid phased mixture, has both a water-soluble component and a water-insoluble component. This type of extraction pulls one or more compounds out of the solid mixture into a solvent of the same polarity (Padias 128-37), leaving behind the compounds that are un-soluble. The method is started by placing a mixture of solid compounds into a container and adding either a liquid non-polar solvent, such as diethyl ether, or a liquid polar solvent, such as acetic acid, to dissolve solutes of the solid mixture. Because “like dissolves in like,” the desired solute is dissolved into the solvent and the undesired solute is left in the solid phase. Through use of vacuum filtration, the solid and liquid can be separated, leaving the solid on the filter paper and the liquid in the container below. Liquid-liquid extraction is a method based on the varying solubilities of different solutes in immiscible solvents (Padias 128-37). Because the solvents are immiscible, they form distinct layers, with the denser layer on the bottom. These layers can be separated through the use of a seperatory funnel which drains the bottom layer into a separate container. This method uses the understanding of partition ratios of solutes to different paired solvents to produce an equilibrium leaning towards one solvent over another, thereby extracting a compound from one liquid to the
other (Padias 128-37). For example, consider a mixture containing two solutes, solute A and solute B, and two immiscible solvents, solvent A and solvent B. If solute A dissolves well into solvent A, but not very well into solvent B, and solute B dissolves well into solvent B but not very well into solvent A, there would be a higher ratio of solute A in solvent A than in solvent B, and a higher ratio of solute B into solvent B than in solvent A. One can then see that, through the use of different solvents, two dissolved solutes can be separated from a mixture. This ratio of a solute concentration to different solvents is defined by K, the distribution constant. Successive filtrations yield’s a higher percentage of products. Chemically active extraction is a highly useful method of separating organic compounds in mixtures (Padias 128-37). Organic acids and bases are soluble in organic solvents, but their corresponding salts are soluble in water. Therefore, if one had a mixture of an acidic organic compound and a basic organic compound in an organic solvent, addition of a base would convert the acidic compound to the corresponding salt, making it water soluble, and the basic compound would be left unreacted. The mixture would then create two layers which could be drained and separated. After separating the two layers, an addition of an acid and an organic solvent would change the salt back to its original form, thus creating an aqueous layer and a layer with the organic solvent and the organic compound. Evaporation of the mixture then leads to isolation of the organic compound.

This experiment uses all three different methods of extraction. Through these methods, it is expected that a pure sample of benzoic acid can be separated from the crude benzoic acid mixture.

Approximately 4 g of crude benzoic acid mixture was weighed on a balance and placed in a 125 mL Erlenmeyer flask. 50 mL of diethyl ether was poured into the flask, along with 3 boiling stones. The mixture was heated on a steam bath until the ether boiled. A Buchner funnel was used to separate the filtrate from the solid precipitate by vacuum filtration. The solid was discarded and the filtrate was placed into a 500 mL separatory funnel. 30mL of 1M NaOH was added to the 500 mL separatory funnel and was swirled and

vented every 10 seconds until the reaction finished. The aqueous layer was drained into a 250 mL beaker. 30mL of 1M NaOH was added again to the 500 mL separatory funnel and was drained into the same 250mL beaker. The mixture in the beaker was put on ice and was acidified by 50 mL of cooled 6 M HCl. A Buchner funnel was used to collect the solid precipitate by vacuum filtration. The weight of the precipitate was recorded and place into a locker for 1 week. Results

4.01 g of crude benzoic acid was obtained in the solid form as a fine brown mixture containing benzoic acid, cellulose and methyl orange. The addition of 50 mL of clear diethyl ether caused the benzoic acid and the methyl orange to dissolve to a brown-yellow liquid. The brown cellulose remained undissolved and in the solid form. Vacuum filtration separated the solid cellulose from the diethyl ether mixture, causing the liquid, containing the benzoic acid and methyl orange, to become a lighter yellow color. There was also orange oil like drops in the diethyl ether mixture. Adding 30mL of NaOH to the mixture resulted in the formation of two distinct layers in the seperatory funnel; a lighter yellow on top containing a larger ratio of methyl orange and a darker yellow layer on the bottom containing a larger portion of benzoic acid. Draining the funnel left the lighter yellow liquid in the funnel. An addition of another 30 mL of NaOH to the funnel caused two layers to once again be formed. The bottom layer was less concentrated than the first attempt and both attempts were combined resulting in a clear yellow liquid. There was no noticeable change in composition or color once the mixture was put on ice. The addition of 50 mL of HCl changed the yellow liquid to a distinct pink liquid containing the acid indicator, methyl orange, and a solid white precipitate, the benzoic acid. Vacuum filtration of the mixture separated the precipitate, which was white with trace amounts of pink, from the liquid, which was a clear pink color. 6.05 g of the precipitate was obtained at the end of the lab. Works Cited http://businessays.net/solids-recrystallization-and-melting-points/